Waterjet propelling apparatus for vessels is configured as a turbopump with an impeller for pressurizing water drawn from a suction port open at the bottom of a vessel, converting into swirling streams, and a diffuser for rectifying the swirling streams into straight streams, to discharge thus obtained waterjets from a discharge part at the stern, thereby propelling the vessel.
Table-1 lists fundamental impeller types and typical characteristics of turbopumps.
TABLE 1Fundamental Impeller Types and Typical CharacteristicsTypesCentrifugalMixed flowAxial flowOutflow directionRadialDiagonalAxialHead providerCF*1CF*1 + VPF*2VPF*2Head, HHighModerateLowDelivery, QSmallModerateLargeSpecific speed, Ns100 to 150350 to 1,1001200 to 2000Meridian contourC1, C2 ofC3 to C6 of FIG. 11C7 of FIG. 11FIG. 11*1CF = centrifugal force, *2VPF = vane's pumping force 
As shown in the Table-1, the impeller of turbopump is classifiable into three fundamental types according to the outflow direction of pumped liquid. In other words, a centrifugal type has an outflow direction substantially perpendicular to the axis of rotational, which is radial; a mixed flow type has an outflow direction diagonal to the axis of rotation; and an axial flow type has an outflow direction substantially parallel to the axis of rotation. In the axial flow type, liquid flows in an axial direction, receiving axial pumping forces from the vanes of the impeller, and obtaining a head principally therefrom. In the mixed flow type, flowing liquid has radial moving components and receives commensurate centrifugal forces, as well as pumping forces from vanes, thereby obtaining a head. In the centrifugal type, liquid flows in radial directions, receiving centrifugal forces, and obtaining a head principally therefrom. Accordingly, in general, the centrifugal type has high head and small delivery. In contrast, the axial flow type has low head and large delivery. The mixed flow type falls somewhere in between.
In this respect, the outflow direction of pumped liquid depends on changes in the radial direction of liquid channels. The radial changes of channels can be seen with ease, by observing a meridian map of the channels, i.e., a meridian channel (hereafter sometimes called “M-channel”).
The meridian map is a rotational mapping of a body of rotation onto a meridian plane (i.e., a plane that includes the axis of rotation). In the case of turbopump, it appears as a meridian contour (hereafter sometimes called “M-contour”), where the impeller and a casing that constitutes a shroud of one or more channels have their inside contours (which actually extend in a circumferential direction with their curvilinear changes) circumferentially projected on a plane including an axis of the impeller, there being manifested an angular change.
The M-contour can be generally specified by a non-dimensional parameter called “specific speed”. The specific speed corresponds to a required number of revolutions (rpm) of a turbopump for delivery of a unit flow rate (1 m3/min) of liquid pumped to a unit head (1 m). Now, letting Q (m3/min) be a delivery flow at a designed number of revolutions N (rpm), and H (m) be a total head, the specific speed Ns of the turbopump can be expressed such that:Ns=N·Q1/2/H3/4.
For conventional turbopumps, FIG. 12 shows a relationship between the specific speed Ns and exemplary M-contours MC1 to MC7. As is apparent from FIG. 12, for the centrifugal type (MC1, MC2) to be large in H and small in Q, the Ns can be as small as ranging approx. 100 to approx. 150, however for the axial flow type (MC7) to be small in H and large in Q, the Ns can be as large as ranging approx. 1,200 to approx. 2,000. For the mixed flow type (MC3 to MC6), the Ns can decrease from approx. 550 to approx. 350, as the outflow direction of pumped liquid approaches (MC3←MC4) a radial direction, or on the contrary can increase from approx. 600 to approx. 1,100, as the outflow direction of pumped liquid approaches (MC5→MC6) an axial direction. M-contours, e.g., MC1 and MC2, of impellers of the centrifugal type define M-channels, e.g., mp1 and mp2, extending in a radial direction at their delivery ends. M-contours, e.g., MC3 to MC6, of impellers of the mixed flow type define M-channels, e.g., mp3 to mp6, diagonal to the axis of rotation at their delivery ends. M-contours, e.g., MC7, of impellers of the axial flow type define M-channels, e.g., mp7, substantially parallel to the axis of rotation at their delivery ends.
Japanese Patent Application Laying-Open Publication No. 11-70894 has disclosed a waterjet propelling apparatus for vessels using an axial flow type of impeller with a cylindrical impeller casing. This waterjet propelling apparatus can discharge a large amount of waterjets with a relatively low pressure, and is suitable for propelling large-scale low-speed vessels.
Japanese Patent Application Laying-Open Publication No. 2000-118494 has disclosed a waterjet propelling apparatus for vessels using a mixed flow type of impeller with a drum-shaped impeller casing. This waterjet propelling apparatus can discharge waterjets higher in pressure, but inferior in flow rate, relative to the use of axial flow impeller, and is suitable for propelling middle-speed vessels small or middle in scale.
Japanese Utility Model Application Laying-Open Publication No. 1-104898 has disclosed a waterjet propelling apparatus for vessels, using a combination of a front stage booster and a mixed flow type of impeller. This waterjet propelling apparatus can discharge boosted waterjets with a fraction of contribution by the booster, and is suitable to middle-speed vessels small or middle in scale and high-speed vessels small in scale.
Japanese Patent Application Laying-Open Publication No. 8-253196 has disclosed a waterjet propelling apparatus of an outboard type using a centrifugal type of impeller. This waterjet propelling apparatus can discharge waterjets still higher in pressure, but still inferior in flow rate, relative to the use of mixed flow impeller, and is suitable to small-scale high-speed vessels.
FIG. 13 shows, in a meridian map, a mixed flow type of impeller IMP-0 used in a conventional waterjet propelling apparatus for vessels. This impeller IMP-0 is configured with a rotary hub 115 in a frustum shape of a right circular cone, and a plurality of rotary vanes 116 wound around the hub 115. The hub 115 has an outer periphery 115a extending from an upstream (i.e., small-diameter end) edge 115b thereof to a downstream (i.e., large-diameter end) edge 115c thereof, at a maintained angle up to a vicinal part 115d to the downstream edge 115c within a range of about 15° to 30° relative to a rotation axis CL of the hub 115, and at a varied angle from the vicinal part 115d within a range of about 0° to 22°. Respective rotary vanes 116 have, as they are in the meridian map, an inner peripheral edge part 116a extending along the hub outer periphery 115a, and an outer peripheral edge 116b extending at a maintained angle within a range of about 0° to 22° relative to the rotation axis CL. This vane configuration improves the head and flow rate of mixed flow impeller to some extent that is yet insufficient for application to high-speed vessels relatively large in scale.
The present invention has been made with the foregoing points in view. It therefore is an object of the invention to provide a waterjet propelling apparatus for vessels applicable even to a high-speed vessel relatively large in scale.